1. Field of the Invention
This invention relates generally to methods for agglomerating or pelletizing mineral ore concentrate. More specifically, this invention relates to methods for agglomerating or pelletizing mineral ore concentrate using water soluble polymers as a dispersion in oil including a water-in-oil emulsion, or as a dry powder. The process of this invention may be used to make conventional pellets, known as acid pellets, or to make flux pellets.
2. Description of the Prior Art
It is customary in the mining industry to agglomerate or pelletize finely ground mineral ore concentrate so as to further facilitate the handling and shipping of the ore. Mineral ore concentrates can include iron oxides, copper oxides, barytes, lead and zinc sulfides, and nickel sulfides. Agglomerates of coal dust and nonmetallic minerals used to make brick or ceramics are also formed. Finished agglomerate forms can include pellets, briquettes, and sinters.
Methods of pelletizing mineral ore concentrate are frequently used in mining operations where the ore is a low grade iron ore, although it may also be utilized with high grade ore. Examples of low grade iron ores are taconite, hematite, and magnetite. Numerous other low grade ores exist wherein pelletizing of the ground particles is beneficial to the handling and shipment of the mineral ore. After the mineral ore has been mined, it is frequently ground and screened to remove large particles which are recycled for further grinding. Typically, an ore is passed through a 100 mesh (0.149 mm) screen. The screened mineral ore is known as a "concentrate". The concentrate may be further processed by flotation to remove up to about 1.5% weight additional silica.
For example, taconite mineral ore concentrate after grinding and screening has an average moisture content of between about 6 to about 11 percent. The moisture content of the mineral ore concentrate can be selectively altered. The moisture content affects the strength of the balls that are formed later in the process and the kinetics of balling as well as the BTUs required to fire them to pellets.
After screening, the mineral ore concentrate is transported on a first conveyor means to a balling drum, balling disc, or another means for balling mineral ore concentrate. Prior to entering the balling means, a binding agent is applied or mixed into the mineral ore concentrate. Commingling the binding agent with the mineral ore concentrate occurs both on the conveyor means and in the means for balling. The binding agent holds the mineral ore concentrate together as balls until they are fired.
Balling drums are apparatus comprising long cylindrical drums which are inclined and rotated. The mineral ore concentrate is simultaneously rotated about the balling drum's circumference and rolled in a downward direction through the drum. In this manner, the mineral ore concentrate is rolled and tumbled together to form roughly spherical-shaped balls. As the balls grow in size and weight they travel down the incline of the drum and pass through the exit of the drum at which point they are dropped onto one or more conveyor means which transports them to a kiln for firing. Inside the balling drum, different factors influence the mechanisms of union of the mineral ore concentrate. These factors include the moisture content of the ore, the shape and size of the mineral ore particles, and the distribution of concentrate particles by size. Other properties of the mineral ore concentrate that influence the balling operation include the mineral ore's wettability and chemical characteristics. The characteristics of the equipment used, such as its size, speed of rotation and angle of the drum with respect to the horizontal plane, can each effect the efficiency of the balling operation. The nature and quantity of the agglomerating or binding agent used in the concentrate is also a factor that determines part of the efficiency of the balling operation.
The formation of agglomerates begins with the interfacial forces which have a cohesive effect between particles of mineral ore concentrate. These include capillary forces developed in liquid ridges among the particle surfaces. Numerous particles adhere to one another and form small balls. The continued rolling of the small balls within the balling apparatus causes more particles to come into contact with one another and adhere to each other by means of the capillary tension and compressive stress. These forces cause the union of particles in small balls to grow in much the same manner as a snowball grows as it is rolled.
After the balling drum operation, the balls are formed, but they are still wet. These balls are commonly known as "green balls" though taconite balls, for example, are usually black in color. Green balls usually have a density of about 130 lb/ft.sup.3 in sizes between about 1/2 inch and about 3/8 of an inch. The green balls are transported to a kiln and heated in stages to an end temperature of approximately 2800.degree. F. After oxidation, fired green balls are denoted as "pellets" and are extremely hard and resist cracking upon being dropped and resist crushing when compressed.
Two standard tests are used to measure the strength of both green balls and pellets. These tests are the "drop" test and the "compression" test. The drop test requires dropping a random sampling of pellets or balls a distance, usually about 18 inches or less, a number of times until the pellets or balls crack. The number of drops to crack each pellet or ball is recorded and averaged. Compression strength is measured by compressing or applying pressure to a random sampling of pellets or balls until the pellet or ball crumbles. The pounds of force required to crush the pellets or balls is recorded and averaged. The drop and compressive test measurements are important because balls, proceeding through the balling apparatus and subsequent conveyor belts, experience frequent drops as well as compressive forces from the weight of others travelling on top of them. Additionally, pellets are also transported by conveyor and are deposited into rail cars and ship holds so that they too require stringent physical characteristics.
The tumble strength of pellets can also be tested. The tumble strength test is designed to measure impact abrasion resistance of pellets. To test tumble strength, equal weight samples of a selected size of pellets, such as 1/2 inch pellets, are rotated in a drum at a standard speed for equal amounts of time. The samples of pellets are then removed from the drums and sized on a 1/4 inch screen. The amount of small particles and fines that pass through the screen is compared between samples. High percentages of fines indicate that, during shipment, the pellets can be expected to deteriorate. A high rate of deterioration during shipment results in higher costs in smelting the pellets and poor blast furnace performance. Tumble test results are also used to calculate a "Q index" i.e., "Quality"-index. The Q-index was derived by the American Society for Testing and Materials (ASTM) and is described in the ASTM publication E279-65T. A high Q-index such as a value of about 94 or greater is an indication that the pellets are impact and abrasion resistant. Alternately, a %-1/4 inch "after tumble test" can be measured and used as a measure of both pellet impact and abrasion resistance.
Thermal shock resistance is another factor which must be taken into consideration in any process for agglomerating mineral ore concentrate. Increases in a ball's thermal shock resistance improve that ball's ability to resist internal pressures created by the sudden evaporation of water when the ball is heated in a kiln. If the ball has numerous pores through which the water vapor can escape or if the rate of water movement to the surface of the balls is enhanced, then thermal shock resistance is improved. If the surface of the ball is smooth, continuous and without pores, or the rate of water movement is too slow, then the ball has an increased tendency to shatter upon rapid heating. This causes a concurrent increase in the amount of "fines" or coarse particles in the pelletized mineral ore. A binder which increases the porosity of balls or which accelerates rate of water movement to the surface improves that ball's ability to resist thermal shock.
Both the binder agent and balling apparatus used to form balls from a mineral ore concentrate can affect the ball size distribution obtained during the balling operation. It is desirable to form balls having a diameter of approximately 1/2 inch. It is also desirable to have a low variation between the diameter sizes of the balls formed during a balling operation. Pellets having a diameter of more than about 1/2 inch are less capable of being reduced in a blast furnace because of their increased surface area. Pellets having a diameter of about 1/2 inch are easily reduced in a furnace and result in fuel efficiency in the operation of the furnace as compared to reducing pellets of larger diameters. Pellets having a size distribution averaging less than 3/8 inch have an increased resistance to gas flow within a furnace. The increased resistance to gas flow decreases furnace productivity and adversely affects the fuel rate consumption of the furnace during operation. Desirable permeability of pellets to gas flow within a furnace is obtained when the pellets are reasonably large, evenly sized, have an approximately even distribution of surface area and provide sufficient porosity. An even distribution of surface area is best obtained starting with spherical balls a compared to balls which vary in their geometrical shapes. The optimum pellet size for furnace operations is between about 3/8 inch and about 1/2 inch in diameter.
Bentonite (montmorillonite) clay is used as a binding agent in the pelletizing operations for mineral ore concentrate such as taconite ore concentrate. Bentonite produces a high strength ball and pellet having acceptable drop strengths, compressive strengths, and a ball having acceptable thermal shock resistance. Bentonite also provides moisture control in the formation of balls made from mineral ore concentrate. Moisture control in the formation of balls is important because the rate of growth of balls increases with increased moisture. This increase in the rate of growth of the balls is due to the increased efficiency of the agglomerate adhesion. Commercially available bentonite has a typical layer structure, a high particle surface area, and a specific affinity for water. Bentonite's ability to act as a binding agent in balling operations for mineral ore concentrates is believed to result from the immobilization of water contained in a mineral ore concentrate. Bentonite is believed to immobilize water in the mineral ore concentrate by absorbing free water into the surface layers of the bentonite clay. The addition of bentonite to a mineral ore concentrate decreases the water available for participating in the balling of the mineral ore concentrate which leads to a desirable retardation in the pellet growth process during the balling operation.
Bentonite has the disadvantage of increasing the silica content of the pellets that are ultimately formed. Bentonite is converted to silica when balls containing bentonite are fired at about 2400.degree. F. or higher. Bentonite also imparts a significant concentration of acidic components to the pellets. Silica decreases the efficiency of blast furnace operations used in smelting of the ore. For this reason bentonite requires a higher energy expenditure than do organic binders in the blast furnace.
The presence of silica and alkalis in pellets of mineral ore concentrate also affects the hot metal quality and furnace operating efficiency during steel production. For these reasons, rigid specifications exist for the presence of these contaminates in pellets of mineral ore concentrates and it is desirable to keep the presence of these contaminates in pellets as low as possible. Specifically, silica separates from the mineral ore in the cohesive zone to form slag. The addition of a 1% concentration of bentonite or 24.4 lbs./tonne provide an undesirable 0.85% silica or silicon dioxide (SiO.sub.2) and alumina or aluminum oxide (Al.sub.2 O.sub.3). This concentration of silica and alumina decreases the iron content of a pellet about 0.6%. Additionally, the quantity of slag is undesirably higher with this concentration of bentonite. An increased quantity of slag within the furnace decreases the productivity and fuel rate consumption of the furnace during the smelting operation. The increase in slag during the smelting operation resulting from the presence of bentonite in the mineral ore concentrate pellets also affects hot metal sulfur control. Other disadvantages of the presence of bentonite in pellets include an increased shipping expense because of the additional weight added to the pellets by bentonite, and an increase in the requirement for limestone and coke during the smelting operation. The additional limestone and coke used during the smelting operation increases costs and reduces the amount of iron ore that can be converted to iron at a constant volume within the blast furnace.
The use of bentonite to form balls of a mineral ore concentrate also adds alkalis which are oxides of sodium, potassium, and zinc. These alkalis are reduced in the stack zone of a blast furnace, descend into the blast furnace and are vaporized and recirculated in the stack zone. The phenomenon occurs with alkalis because of the low boiling points of these metals. The presence of alkalis in the blast furnace causes both the pellets and coke to deteriorate and form scabs on the furnace wall which increase the fuel consumption rate and decrease the productivity of the smelting operation. The decrease of productivity of the smelting operation results from a decrease in the gas permeability of the pellets. When the scabs become too large to adhere to the walls of the blast furnace, the scabs fall from the walls and cause a burning of tuyeres, a cooling of the hot metal, and a disruption of the smelting operation. The disruption of the smelting operation can result in quality control problems during the production of steel, as well as in safety problems. An additional safety problem that occurs with the use of high concentrations of bentonite in the formation of pellets is an increased exposure to asbestos. Bentonite contains asbestos which can be carried through the process to plant effluent water.
Other binding agents have proven to be useful as binders besides bentonite. These agents or "ore binding polymers" include organic binders such as poly(acrylamide), polymethacrylamide, carboxymethyl cellulose, hydroxyethyl cellulose, carboxyhydroxyethyl cellulose, poly(ethylene oxide), guar gum, and others. The use of organic binders in mineral ore balling operations is desirable over the use of bentonite because organic binders do not increase the silica content of the bound material and they improve the thermal shock resistance of the balls. Organic binders burn out during ball firing operations and cause an increase in the porosity of the pellets. Firing conditions can be modified to improve the mechanical properties of pellets for organic binder systems.
Some organic binders used in mineral ore balling operations are dissolved in an aqueous solution which is sprayed onto the mineral ore concentrate prior to entering the balling drums or other balling means. This application of an aqueous solution increases the moisture content above the natural or inherent moisture content of the mineral ore concentrate which requires a greater energy expenditure during the firing operation of the balls. This increased moisture content also causes an increased likelihood of shattering due to inadequate thermal shock resistance during firing. Ball formation is improved with the use of organic binders, but the drop strength and compression strength of the ball and pellet are frequently below that desired or achieved with bentonite.
Other binders commonly used for agglomerating mineral ore concentrate include a mixture of bentonite, clay and a soap, Portland cement, sodium silicate, and a mixture of an alkali salt of carboxymethylcellulose and an alkali metal salt. The agglomerates made from these binding agents frequently encounter the problems described above of insufficient ball strength or insufficient porosity for the rapid release of steam during induration with heat. Additionally, these binding agents are usually applied to a mineral ore concentrate in aqueous carrier solutions which increase both the amount of energy required to fire the balls and the incidence of shattering due to inadequate thermal shock resistance.
U.S. Pat. No. 3,893,847 to Derrick discloses a binder and method for agglomerating mineral ore concentrate. The binder used is a high molecular weight, substantially straight chain water soluble polymer. This polymer is used in an aqueous solution. The polymers disclosed as useful with the Derrick invention include copolymers of acrylamide as well as other polymers. The Derrick invention claims the use of polymers in an "aqueous" solution. The use of water as a carrier solution for the binding agents increases the moisture of the agglomerate or balls that are formed. The higher moisture content increases the energy required to fire the balls and can increase the rate of destruction of the balls during induration due to the rapid release of steam through the agglomerate.
The use of organic polymers, regardless of the molecular weight of the organic polymers or the form in which they are applied to a mineral ore concentrate, can result in formation of balls having dissimilar geometric shapes. The application of organic polymers in solution, water-in-oil emulsion, and dry powder forms in conjunction with inorganic salts such as sodium carbonate have resulted in the formation of non-spherical balls. The formation of non-uniform, non-spherical balls results in a greater variation in the surface area of the balls and therefore, the pellets, which results in undesirable high levels of fines being generated during formation of pellets and moreover, uneven reduction of the pellets in the furnace during the smelting operation. Higher levels of fines are also generated when small portions of roughened ball or pellet surface are abraded during transport which then produce undesirable dust within the furnace and a the blast furnace.
The non-uniform, non-spherical formation of balls resulting from the use of organic polymer binding systems and inorganic salts results from an undesirable alteration in the ball growth process due to the presence of the inorganic salt and its interaction with surface moisture. Moisture control is important because the rate of ball growth increases with increased moisture.
Two articles by Clum et al. entitled, "Possible Binders for Pelletizing of Magnetic Taconite Concentrates", Mining Engineering 30 (1) page 53, 1978, and "Substitutes For Western Bentonite In Magnetic Taconite Pellets", Society of Mining Engineers of AIME, preprint 76-B-11, 1976, relate to balls of magnetite concentrate using binders of: Wisconsin clay, hydroxyethyl cellulose, poly(ethylene oxide), and a guar gum derivative. The binder systems used in the pelletizing operations of these articles are undesirable because the binder systems utilize an undesirably high concentration of polymer. Additionally, the distinct components result in increased manufacturing difficulties, expenses of manufacturing, and decreased predictability in the performance of the binder system with various mineral ore concentrates. The decrease in predictability of the binder system with various mineral ore concentrates results from the increased complexity of the binder system resulting from the introduction of additional components to the balling operation. The high concentration of polymer in the binder system used in these articles results in an increased cost that can make using these articles undesirable over other commercially available binder systems.
Organic polymers have been used as bentonite extenders wherein the polymers themselves do not significantly add to the strength of the resulting pellets. Additionally, various synthetic and natural resins and modified resins have been used in conjunction with bentonite to pelletize mineral ores. As disclosed in an article by Das Gupta et al., "Additives To Increase Bentonite Effectiveness In Iron Ore Pelletizing", Society of Mining Engineers of AIME, preprint 78-B-97 at page 1, the use of polymers with bentonite has resulted in less than desirable (1) ball formation or (2) pellet reducibility and behavior in a blast furnace. Additionally, this article reports undesirable economic factors resulting from high concentration of the resins required to effectively ball a mineral ore concentrate.
The industry is lacking a method for agglomerating mineral ore concentrate utilizing a two component low moisture polymer binder system, wherein moisture control is provided during ball formation and wherein the balls and pellets formed from the mineral ore concentrate have high mechanical strength properties.